1. Field of the Invention
The present invention relates to a non-linear photoelectric converting apparatus in a photoelectric converting device of an accumulation type such as a CCD or the like for converting a photosignal into an electric signal in which the gamma characteristic of the output electric signal for the input photo signal can be set to .gamma.&lt;1.
2. Related Background Art
Solid state image pickup devices such as CCD and the like consist of semiconductor and, therefore, have various advantages such that they are small-sized and light-weight, and electric power consumption is small, reliability is high, and the like. Also, these devices sequentially read out signals generated in pixels arranged accurately like a mosaic, so that graphic distortion is very small and the like. For these reasons, such solid state image pickup devices have been used widely in recent years. In most such devices, the gamma characteristic .gamma. is nearly equal to 1 and the gradations of the input image and output image are almost equal and excellent linearity is obtained. On the contrary, however, there are the following drawbacks. The dynamic range is narrower than that of the silver salt film. It is difficult to obtain a video signal such that images in the natural world and the like can be reproduced with a high degree of clearness and brightness from high luminance to low luminance. It is likely for the dark portion to be blacked out and the bright portion to be saturated.
In addition, since the dynamic range which can be expressed by the display device is generally narrow, in order to reduce the burden of signal processes, the .gamma. conversion such as logarithm conversion or the like needs to be performed in the gamma characteristic and it is necessary to convert the luminance level into the compressed signal and to execute the signal processes. However, even if the processes such as .gamma. conversion and the like are performed at the post stage by the CCD or the like, the dynamic range of luminance information of the image signal which is derived from the CCD or the like is narrow, so that the effective advantage cannot be obtained. Therefore, there is the drawback such that in the case of compressing the image signal by the .gamma. conversion at the post stage of the image pickup device, an optical image of a relatively wide dynamic range cannot be substantially handled.